<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>任务2</title>
    <style type="text/css">
        p{
            font-weight: bold;
        }
        .title { background-color: red;
            }
        #test {text-align: center; }
    </style>
</head>
<body>
<p id='test' class='title'>Hello world!</p>
</body>
</html>